This is a simple audio transformer mixer than I made from a transformer that had two input channels.
This circuit is risky because there are mutual voltages between the two inputs. The first input coil in transformer is getting induced current from the second input coil and vice versa. That means the left input can receive current from right input even if they are completely disconnected. If you Google transformer models on the internet you will find that there are voltage sources at the input of transformers that are induced currents coming from transformer outputs.
However, I decided to take this risk and build this circuit hoping that the input impedance (resistance) will reduce the impact of those mutually induced currents.
You can see this circuit working in the videos below:
This circuit mixes the left and right of two stereo channels because I did not have two music sources.
If you are going to build this circuit than keep in mind that you should check the connection between transformer nodes to ensure that the left channel/first channel is not connected to right channel/second channel. This will damage your audio output. You need to identify the ground of each channel and leave the each of two grounds disconnected because one of the outputs could be a differential output and shorting it to ground might damage to audio amplifier.
You can see in the photo below how I attached the wires.
Making holes in this box was easy because the box is not made from a hard and strong material: